Tech Stuff

Noise/vibration/harshness (Part II): Chasing the irritant gremlins

In part one of this article in our January/February 2013 issue we covered various sources of vibrations and noises that may be described by your customers. We also covered radial force variation (RFV) and its causes and solutions. In this article, we will take a closer look at the types of vibration plus harshness complaints and solutions.

It may seem obvious, but the wheel must be properly centered on the hub. If the wheel is lug centric, the position of the wheel studs dictate centering. If hub centric, the center of the hub engages to the wheel’s center hole for proper wheel position.
<p>It may seem obvious, but the wheel must be properly centered on the hub. If the wheel is lug centric, the position of the wheel studs dictate centering. If hub centric, the center of the hub engages to the wheel’s center hole for proper wheel position.</p>

Vibration is a shaking or trembling that can be felt by the customer when an object/component moves back and forth or up and down consistently. Abnormal vibrations usually occur under certain vehicle operating conditions.

There are three types of vibrations:

Free vibration — a vibration that continues after the cause has been removed. For example, a tire hitting a pothole will continue to vibrate after the initial impact has passed.

Forced vibration — a vibration that only occurs as long as the force that initiated the vibration remains. For example, an unbalanced driveshaft only causes a vibration as long as it is rotating. Another example would be an unbalanced tire, which would stop vibrating when it stops rotating. Forced vibrations are the most common type dealt with in automotive applications.

Torsional vibration — vibration caused by a constant twisting force that is felt in the floor and seats of the vehicle. This type of vibration is most noticeable during hard acceleration and is amplified by the application of torque.

Under normal circumstances, a rotating component will not produce a noticeable vibration. However, if the component has improper weight distribution (imbalance) or is rotating in an eccentric pattern (out-of-round or bent), then a vibration may be produced. If the frequency and amplitude of the vibration can be measured, then those characteristics along with the vehicle speed and engine rpm at which the vibration occurs, can be matched with components that would likely cause the vibration at that particular speed. This procedure can help find the source quickly and accurately.

Vibrations are often noticed in a component far removed from where they are generated. This is called transfer path or telegraphing. For example, an out-of-balance front tire and wheel assembly may result in a noticeable steering wheel shake. In this case, we would call the wheel and tire assembly the origin (or originator), the steering linkage the conductor, and the steering wheel the reactor.

Damaged or worn engine and body mounts or a grounded exhaust hanger are components that could transmit (conduct) a normal engine vibration (originator) into the passenger compartment (reactor) as an NVH concern.

Harshness

Harshness is a concern that is related to the customer’s perception or expectation of a vehicle’s ability to absorb vibrations caused by road imperfections.

Harshness is usually the result of:

• Deterioration of vehicle components — such as worn or damaged suspension components that cannot move within their normal range of motion, or that have lost their isolating grommets or bearings.

• Modification of original equipment — such as over-sized tires or heavy-duty springs and shocks.

• Improper tire inflation — over– or under-inflated tires can cause a harsh ride.

Advanced electronic listening devices, such as the ChassisEAR, can be used to quickly identify a noise and its location under the chassis while the vehicle is being road-tested.

These versatile devices can identify the noise and location of bad wheel bearings and various problems in the differential transmission, CV joints, brakes, leaf and coil springs, transfer case, pinion bearings or carrier bearings.

For example, the ChassisEAR has a six-position input selection control switch with six microphone clamps that are attached to 16-foot leads. The leads are secured to the vehicle with clamps and Velcro ties.

The ChassisEAR provides instant comparisons between any of the six channels during the road test. The unit is equipped with headphones that block out surrounding noises.

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